Auto merge of #54782 - pnkfelix:issue-54556-semi-on-tail-diagnostic, r=nikomatsakis

NLL: temps in block tail expression diagnostic

This change adds a diagnostic that explains when temporaries in a block tail expression live longer than block local variables that they borrow, and attempts to suggest turning the tail expresion into a statement (either by adding a semicolon at the end, when its result value is clearly unused, or by introducing a `let`-binding for the result value and then returning that).

Fix #54556

src/test/ui/issues/issue-18783.nll.stderr

@@ -11,7 +11,7 @@ LL |     c.push(Box::new(|| y = 0));
    |                     second mutable borrow occurs here
 LL | //~^ ERROR cannot borrow `y` as mutable more than once at a time
 LL | }
-   | - first borrow later used here, when `c` is dropped
+   | - first borrow might be used here, when `c` is dropped and runs the destructor for type `std::cell::RefCell<std::vec::Vec<std::boxed::Box<dyn std::ops::FnMut()>>>`
 
 error[E0499]: cannot borrow `y` as mutable more than once at a time
   --> $DIR/issue-18783.rs:26:29
@@ -26,7 +26,7 @@ LL |     Push::push(&c, Box::new(|| y = 0));
    |                             second mutable borrow occurs here
 LL | //~^ ERROR cannot borrow `y` as mutable more than once at a time
 LL | }
-   | - first borrow later used here, when `c` is dropped
+   | - first borrow might be used here, when `c` is dropped and runs the destructor for type `std::cell::RefCell<std::vec::Vec<std::boxed::Box<dyn std::ops::FnMut()>>>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-47646.stderr

@@ -3,9 +3,15 @@ error[E0502]: cannot borrow `heap` as immutable because it is also borrowed as m
    |
 LL |     let borrow = heap.peek_mut();
    |                  ---- mutable borrow occurs here
-...
+LL | 
+LL |     match (borrow, ()) {
+   |           ------------ a temporary with access to the mutable borrow is created here ...
+LL |         (Some(_), ()) => {
 LL |             println!("{:?}", heap); //~ ERROR cannot borrow `heap` as immutable
    |                              ^^^^ immutable borrow occurs here
+...
+LL |     };
+   |      - ... and the mutable borrow might be used here, when that temporary is dropped and runs the destructor for type `(std::option::Option<std::collections::binary_heap::PeekMut<'_, i32>>, ())`
 
 error: aborting due to previous error